linux-kernel - Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR

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Date:   Thu, 23 Jul 2020 15:04:13 -0400
From:   Waiman Long <longman@...hat.com>
To:     peterz@...radead.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Anton Blanchard <anton@...abs.org>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for
 SPLPAR

On 7/23/20 2:47 PM, peterz@...radead.org wrote:
> On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote:
>> BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch?
>> I will have to update the patch to fix the reported 0-day test problem, but
>> I want to collect other feedback before sending out v3.
> I want to say I hate it all, it adds instructions to a path we spend an
> aweful lot of time optimizing without really getting anything back for
> it.

It does add some extra instruction that may slow it down slightly, but I 
don't agree that it gives nothing back. The cpu lock holder information 
can be useful in analyzing crash dumps and in some debugging situation. 
I think it can be useful in RHEL for this readon. How about an x86 
config option to allow distros to decide if they want to have it 
enabled? I will make sure that it will have no performance degradation 
if the option is not enabled.

Cheers,
Longman